In the field of graphic arts, a printing plate is produced using a set of color separations of a color original which are prepared using lithographic films. In general, color proofs are prepared from color separations in order to inspect for errors in color separation and to check the need for color correction and the like before final printing (practical printing operation). Color proofs are required to realize high resolution enabling accurate half tone reproduction and high processing stability. To obtain color proofs close to actual prints, it is desirable for the materials of color proofs to be the same as those used on press, i.e., the same paper as the base material and the same pigments as the coloring materials. There is a higher demand for a dry process involving no processing solutions for the preparation of color proofs.
With the recent spread of computerized systems in prepress work, recording systems for preparing color proofs directly from digital signals have been developed. Such computerized systems, particularly contemplated for preparing high quality color proofs, are generally capable of reproducing dot images at 150 lines or more per inch. In order to obtain high quality proofs from digital signals, a laser beam is used as a recording head, which is capable of modulation according to digital signals and focusing into a small spot diameter. Hence it is demanded to develop image forming materials that exhibit high sensitivity to laser light and high resolution enabling reproduction of highly precise dot images.
Image forming materials known useful in image transfer methods using laser light include a thermal melt transfer sheet, which comprises a support, a light-heat conversion layer capable of absorbing laser light to generate heat, and an image forming layer having a pigment dispersed in a heat fusible matrix (e.g., a wax or a binder) in the order described (JP-A-5-58045). In the image forming method with the use of such a thermal transfer sheet of this type, the laser-irradiated area of the light-heat conversion layer generates heat to melt the image forming layer, and the molten part of the image forming layer is transferred to the image receiving sheet laminated on the transfer sheet, thereby forming an image on the image receiving sheet.
JP-A-6-219052 teaches a heat transfer sheet comprising a support, a light-heat conversion layer containing a light-heat converting substance, a heat release layer as thin as 0.03 to 0.3 μm, and an image forming layer containing a colorant in this order. In the case of this heat transfer sheet, the heat release layer reduces its bonding strength between the image forming layer and the light-heat conversion layer upon being irradiated with laser light. As a result, a high precision transfer image is formed on the image receiving sheet provided on the heat transfer sheet. This image formation method using the above-described heat transfer sheet utilizes so-called “ablation”. That is, a laser-irradiated area of the heat release layer decomposes and vaporizes, resulting in reduction of the strength bonding the image forming layer and the light-heat conversion layer in that area. As a result, the image forming layer of that area is transferred to the image receiving sheet laminated thereon.
These imaging methods are advantageous in that images can be formed on printing paper having an image receiving layer (adhesive layer) and that a multicolor image can easily be obtained by successively transferring images of different colors onto the same image receiving sheet. The method utilizing ablation is particularly advantageous for ease of forming a highly precise image and is useful to prepare color proofs (direct digital color proofs: DDCPs) or precise mask images.
With the spread of desk-top publishing (DPT) work, printing companies adopting a computer-to-plate (CTP) system have a strong demand for a DDCP system, which eliminates the need of intermediate film or plate output as has been involved in traditional analog proofing. In recent years, DDCPs with higher quality, higher stability, and larger sizes have been demanded as good approximations to the final prints.
Laser thermal transfer systems are capable of image formation at high resolution. Options include (1) laser sublimation, (2) laser ablation, and (3) laser melt.
Using any of the above-described systems, it is required that a multicolor image forming material comprising a heat transfer sheet and an image receiving sheet shows a high sensitivity of a recorded image and little change in color hue before and after exposure. It is also required to improve the stability with time of a liquid coating composition for the light-heat conversion layer.
In recent years, multibeams of laser light using a plurality of laser beams are employed for shorten recording time in image recording with laser-irradiation. In the case of recording with multibeam laser light by using an existing heat transfer sheet, it is sometimes observed that the transfer image formed on the image receiving sheet shows an only insufficient image density. A serious lowering in the image density arises observed particularly in high-energy laser recording. As the results of studies by the present inventors, it is found out that a lowering in image density is caused by uneven transfer occurring in high-energy laser irradiation.
In the case of recording an image by laser irradiation, there arises another problem that a light-heat converting substance contained in the light-heat conversion layer or its decomposition product migrates into the image forming layer and then is transferred together with the image forming layer, thereby worsening the color hue of the transfer image thus formed.
It is also required to solve these problems.